# slope ## Definition of Slope

The slope of a line is the ratio of the amount that y increases as x increases some amount. Slope tells you how steep a line is, or how much y increases as x increases. The slope is constant (the same) anywhere on the line.

slope, Numerical measure of a line’s inclination relative to the horizontal. In analytic geometry, the slope of any line, ray, or line segment is the ratio of the vertical to the horizontal distance between any two points on it (“slope equals rise over run”). In differential calculus, the slope of a line tangent to the graph of a function is given by that function’s derivative and represents the instantaneous rate of change of the function with respect to change in the independent variable. In the graph of a position function (representing the distance traveled by an object plotted against elapsed time), the slope of a tangent line represents the object’s instantaneous velocity.

## involute

involute, of a curve C, a curve that intersects all the tangents of the curve C at right angles.

To construct an involute of a curve C, use may be made of the so-called string property. Let one end of a piece of string of fixed length be attached to a point P on the curve C and let the string be wrapped along C. Then, as the string is unwrapped, being held taut so that the portion of the string that has been unwrapped is always tangent to C, the locus of the free end of the string is an involute of C. With the same point of attachment P, different involutes of C are obtained by using pieces of string of different lengths.

## curvature

curvature, in mathematics, the rate of change of direction of a curve with respect to distance along the curve. At every point on a circle, the curvature is the reciprocal of the radius; for other curves (and straight lines, which can be regarded as circles of infinite radius), the curvature is the reciprocal of the radius of the circle that most closely conforms to the curve at the given point.

If the curve is a section of a surface (that is, the curve formed by the intersection of a plane with the surface), then the curvature of the surface at any given point can be determined by suitable sectioning planes. The most useful planes are two that both contain the normal (the line perpendicular to the tangent plane) to the surface at the point (see figure). One of these planes produces the section with the greatest curvature among all such sections; the other produces that with the least. These two planes define the two so-called principal directions on the surface at the point; these directions lie at right angles to one another. The curvatures in the principal directions are called the principal curvatures of the surface. The mean curvature of the surface at the point is either the sum of the principal curvatures or half that sum (usage varies among authorities). The total (or Gaussian) curvature (see differential geometry: Curvature of surfaces) is the product of the principal curvatures.

## line

line, Basic element of Euclidean geometry. Euclid defined a line as an interval between two points and claimed it could be extended indefinitely in either direction. Such an extension in both directions is now thought of as a line, while Euclid’s original definition is considered a line segment. A ray is part of a line extending indefinitely from a point on the line in only one direction. In a coordinate system on a plane, a line can be represented by the linear equation ax + by + c = 0. This is often written in the slope-intercept form as y = mx + b, in which m is the slope and b is the value where the line crosses the y-axis. Because geometrical objects whose edges are line segments are completely understood, mathematicians frequently try to reduce more complex structures into simpler ones made up of connected line segments.